Glass and silicon oxide are widely used substrates for biosensors, clinical immunoassay diagnostics, and cell culture (Ratner, Schoen et al. 1996) and as solid supports for the synthesis of peptides, carbohydrates, and DNA (Seeberger and Haase 2000). The modification of silicon oxide to modulate protein and cell interactions has proven to challenging for a number of technical reasons: 1) the formation of silane self-assembled monolayers (SAMs), the most common route to functionalize glass and other metal oxides, is complicated by the sensitivity of most silanes to humidity and their propensity to form polymeric multi-layers (Wasserman, Tao et al. 1989; Ulman 1996). 2) As is typical to most “grafting to” approaches, the passivation of silicon oxide by grafting polyethylene glycol (PEG) to the surface using silane chemistry (Emoto, Harris et al. 1996; Yang, Galloway et al. 1999) does not provide a high surface density of PEG due to the excluded volume effect (Knoll and Hermans 1983). Consequently, grafted PEG coatings on glass decrease the adsorption of proteins, but do not reduce their adsorption below the nominal limit of several ng/cm2 (Zhu, Jun et al. 2001). Several approaches such as the sequential grafting of PEGs of different chain lengths (Nagasaki, Ishii et al. 2001) and cloud-point grafting of PEG (Kingshott, Thissen et al. 2002) have been taken to solve this problem, but only with limited success. 3) It is also difficult to stamp silanes onto glass with the ease and reproducibility with which alkanethiols can be patterned by micro-contact printing and other soft lithography methods so that the patterning of PEG on to glass by soft lithography has only been marginally successful (Xia, Mrksich et al. 1995; StJohn and Craighead 1996).
For the detection of a variety of biological molecules such as protein, RNA, and DNA in complex biological fluids, the minimization of non-specific protein binding plays a very important in improving the detection limit and sensitivity. The reduction of adsorption of protein and other biomolecules is important for the development of interfacial sensors for two reasons: first, for the broad class of sensors that are label-free, i.e., in which the binding event is directly transduced as the detected signal (e.g., surface plasmon resonance (SPR) spectroscopy, localized or nanoparticle-based surface plasmon resonance (nanoSPR), surface enhanced Raman scattering (SERS), ellipsometry, gravimetric sensors such as quartz-crystal microbalance dissipation (QCM-D) and surface acoustic wave (SAW) sensors, etc.) reduction of protein adsorption to ultra-low levels (<1 ng/sq. cm) is critical to generate a high signal-to-noise ratio (SNR) by reducing the noise due to adventitious adsorption. For the class of interfacial sensors that use a label to generate the detected signal, the elimination of background adsorption is similarly important to reduce noise. Finally, for the class of sensors that incorporate an amplification step prior to or during generation of the detected signal, the effective elimination of adventitious adsorption or binding of biomolecules or other reagents is critical, as adventitiously bound molecules can be amplified, so that the increase in signal (S) afforded by the amplification step is in many cases compromised by the concomitant amplification of the background noise (N), so that the gains in SNR are modest, at best.
The increasing technological push towards ultra-sensitive detection in biomolecular arrays—DNA, protein and carbohydrate—similarly requires extremely low background signals so that a high SNR can be attained (Zhu and Snyder 2003). However, most commercially available chemical surface modifications usually have high auto-fluorescence or non-specific binding of reagents and analytes. This issue is increasingly crucial when the spot size of commonly used microarrays becomes smaller and smaller, even down to the sub-micron length scale. Although some of the current surface modification techniques work well for microarrays (Zhu and Snyder 2003), the routine use of micro- and nano-arrays for biomolecules still poses substantial challenges in engineering a detection system that is capable of resisting non-specific adsorption of biomolecules down to the pg/cm2 level and allows direct detection of analytes without elaborate and expensive amplification techniques.